2D sprite viewport: snapshot in, pixels out.
The UI builds a [FrameInput] per frame (plain data, rebuilt
during composition) and mounts [Viewport2d] as a Repose view, which
draws the snapshot through a dp-space [FitMode] and reports pointer
picks back in world coords. Unit discipline lives here, once:
games work in world/dp units and do not touch physical px.
Camera state lives in Repose signals, not in the renderer.